The Assignee of the present application is the manufacturer of a rapid illumination and weather testing apparatus trademarked "Xenotest 1,200".RTM., and described, for example, in literature material D310 561/681. The apparatus permits checking of various and highly different materials with respect to resistance to fading due to light and weather, or other environmental effects. For example, paints, lacquers and varnishes can be tested with respect to color fidelity and maintenance, and also with respect to their mechanical and overall technological behavior. The apparatus has an illumination device with three radiation sectors, which include selectively reflective mirrors, reflecting UV radiation, and being transparent to IR radiation. The selective mirrors extend radially from a common axis outwardly, separating the respective sectors. A IR absorber is located between two each of selectively reflective mirrors of adjacent sectors.
Each one of the three sectors has a xenon radiation source associated therewith to provide the required radiation. This filter-radiation arrangement is surrounded by a quartz inner cylinder with a selectively reflective layer for IR radiation, but passing UV and visible spectral components. A water jacket follows the inner cylinder. The water jacket absorbs long wavelength IR radiation. A quartz outer cylinder, and, eventually, a jacket made of UV special, or window glass, surrounds the structure.
The filter system provides radiation with an energy distribution which very closely matches that of the radiation derived from sunlight. The filtering system largely filters undesired IR components by absorption, and permits passage, selectively, in the short-wave region of a high proportion of radiation to reach the test samples located about the illumination source.
U.S Pat. No. 3,686,940, Kockott, the disclosure of which is incorporated by reference herein and which patent is assigned to an associated organization of the Assignee of the present application, describes a testing apparatus with selective mirrors for removing infrared radiation. The structure has a plurality of eccentrically located radiation sources. A cylindrical mirror is provided which selectively reflects the IR component of the radiation, but is transparent for visible and UV spectral components. Mirrors which are selectively reflective for visible and UV components of the radiation, but passing IR components, are located between the radiation source and the cylindrical mirror. This arrangement permits elimination of short-wave IR radiation, which cause heat, without essentially attenuating the UV radiation.
It has been found that some IR filters, and particularly KG filters, that is, heat absorption filters, when also subjected to UV radiation, change their filtering characteristics. Such filters are particularly desirable and useful to test for resistance to fading, and light effects. These filters have the tendency to change their filtering limit in the UV region towards longer wavelength to such an extent that the desired radiation spectrum is undesirably influenced thereby. The change of the UV limitation is also referred to as aging of the filter.